1. Field of the Invention
The present invention relates to a sheet feeding apparatus and an image forming apparatus, and particularly to a sheet feeding apparatus and Invention
The present invention relates an image forming apparatus equipped with a large capacity sheet accommodating portion and a tray which can control lifting and lowering.
2. Description of the Related Art
In recent years, image forming apparatuses such as copying machines and printers equipped with a large capacity sheet accommodating portion capable of replenishing to one tray a large quantity of sheets as many as several thousands of sheets have been increasing. Such large capacity accommodation portion is frequently equipped with a tray which is capable of lifting and lowering by driving from the sheet accommodating portion body.
With the sheet accommodating portion equipped with the tray, when sheet stacks are replenished on the tray and the sheet accommodating portion is set to the main body, the tray then goes up. When an uppermost surface of the sheet stack is detected by a paper surface position detecting unit, tray stopping control is executed. By this operation, the uppermost surface of the sheet stack becomes as the feeding position.
In many cases, such sheet accommodating portion meets with a plurality of sheet sizes. In this case, when sheets smaller than the maximum size capable of being accommodated in the sheet accommodating portion are replenished, a space free from sheet stack is generated on the tray
It frequently occurs that a paper bag for spare sheets or a surplus sheet stack (hereafter referred to as foreign object) is left on this empty space. When the tray goes up while a foreign object is being placed on the tray, this foreign object may result in damage of the sheet accommodating portion apparatus.
This will be described referring to FIG. 9. FIG. 9 is a drawing for describing a problem due to the foreign object based on conventional configuration. For example, with a tray 61 as illustrated in FIG. 9A, a paper surface position detecting unit (not shown) for detecting upper surface of a sheet stack 50 being set on the tray 61 is usually provided. When upper surface of the sheet stack is detected by the paper surface position detecting unit, lifting of the tray 61 stops.
With this configuration, when a foreign object 90 is placed on the tray 61 and height of the foreign object 90 is higher than that of the sheet stack 50, as illustrated in FIG. 9A, the foreign object collides with uppermost part of an apparatus body 60 before the uppermost surface of the sheet stack 50 is detected by the paper surface position detecting unit.
This occurs similarly when, as illustrated in FIG. 9B, height of the foreign object 90 on the tray 61 is lower than that of the sheet stack 50. In other words, as the sheet is being fed, the tray 61 goes up by as much as lowering of upper surface position of the sheet stack 50. With this state, if upper surface position of the sheet stack 50 becomes lower than upper surface position of the foreign object 90, the foreign object 90 will collide with the uppermost part of the apparatus body 60.
In view of such problems, there is an apparatus which detects a foreign object on a tray, as described by, for example, Japanese Patent Application Laid-Open No. 2003-246470.
According to this proposal, holes are provided to a plurality of locations on the tray capable of lifting and lowering, and sensors are disposed respectively so as to cause a contact to protrude from these holes, when the tray is located at the lowest limit. With this configuration, when an object is placed so as to cover holes on the tray, the object can be detected by the contacts protruded from each of holes. When an object is detected by sensors provided at the position where a sheet is not to be originally placed, it can be determined that a foreign object exists.
However, with above-mentioned unit, in order to detect a foreign object on a sheet, the tray should be lowered to the lowest limit of lifting and lowering range whenever sheets are replenished. Therefore, when sheets are consumed thoroughly, the tray is lowered down to the lowest limit, it takes longer time before the tray is lowered completely, and the user have to wait during this period of time.
Further, according to the composition for detection of a foreign object, which is described by Japanese Patent Application Laid-Open No. 2003-246470, area of the tray covered by sheets replenished depends on the size of sheets. Therefore, a setting as to that touching of which sensor flag is determined to be foreign object should be made differently for every sheet size. In addition, if sheet size is set erroneously, it is probable that even a normal sheet is detected as a foreign object.